Keep a submodule's own .git/config out of the build context (closes #88)

A submodule that keeps its own `.git` directory, instead of one under `.git/modules/`, still shipped `sub/.git/config` into the build context, credential included. Both git patterns in the canonical `.dockerignore` now start with `**/`: `**/.git/config` and `**/.git/modules/**/config`.

A submodule whose name has a `config` segment (`config`, `deploy/config`, `config/lib`) still loses its whole git directory, because the pattern also matches that directory, and Go's version stamping then fails the build loudly. The file records this as a known gap with the way around it, `git submodule add --name`; closing it needs a wildcard re-include that makes every build walk excluded directories. `prompts/REPO_POLICIES.md` and both checklists say the same.

Model: opus-5-5

.dockerignore

@@ -18,9 +18,16 @@
 # does not need .git/config; that file can hold a credential, such as a
 # password in a remote URL or the token the CI checkout step stores there.
 # Each submodule keeps a config with the same exposure in its git directory
-# under .git/modules/, nested again for a submodule's own submodules.
-.git/config
-.git/modules/**/config
+# under .git/modules/, nested again for a submodule's own submodules, or in
+# its own .git directory when it keeps one.
+# KNOWN GAP: a submodule whose name has a `config` segment (`config`,
+# `deploy/config`, `config/lib`) loses its whole git directory, because
+# `**/.git/modules/**/config` also matches that segment's directory
+# under .git/modules/. Go's version stamping then fails the build;
+# nothing leaks. Name such a submodule without that segment:
+# `git submodule add --name`.
+**/.git/config
+**/.git/modules/**/config
 
 # Agent scratch: one full checkout of the repo per in-flight agent.
 # Anchored because it occurs once where agents run at the repo root.